Conventional low cost RF EAS and multibit chipless ID tags are fundamentally limited by their linear nature. They are composed of simple passive inductors, capacitors and resistors that resonate at the reader output frequency when that output frequency matches the resonant frequency of the tag. Tag detection is performed by detecting the disturbance in the oscillating field caused by the presence of the resonating tag (which couples to the reader field by mutual inductance, as in two loosely coupled transformer coils, and causes a change in the impedance of the tag detection circuit at the resonant frequency). This means that the tag resonance signal and the reader output are at the same frequency. Therefore, the detection efficiency and read range can be limited by the signal to noise ratio of the small tag signal with respect to the large reader signal. In some instances, these tags are read by pulsing the reader RF source, then listening for the ringing of the tag oscillator as the resonance decays.
Significant improvements in tag signal-to-noise, reduced error rates, and read range (the distance between a tag and reader) can occur through frequency dividing, multiplying, mixing or shifting in a tag. In this case, the reader puts out a central frequency that excites the tag circuit, such as the nationally and internationally recognized, relatively high field strength but low bandwidth carrier signals at 13.56 MHz. The tag then couples some of this energy into a frequency away from the central reader signal. The reader can then more easily filter out the large drive signal and more easily detect the different frequency “sideband” signal from the tag.
A direct way to get frequency shifts is to include a simple non-linear device into a simple LC circuit. Generally speaking, the introduction of any nonlinear circuit element will lead to the generation of harmonics of the carrier frequency and/or allow the resonant coupling of energy into the tag at frequencies away from the carrier frequency (e.g., tag resonance=the carrier frequency for generation of higher harmonics of the carrier frequency, or for the generation of a subharmonic at half the carrier frequency, tag resonance=½ of the reader signal frequency). A nonlinear device also can allow for mixing multiple incident signal frequencies to produce new spectral components or sidebands. Diodes have been used for these purposes, to produce RF and microwave shifted spectrum frequency tags with enhanced signal-to-noise, read-range and/or a lower false alarm rate than linear capacitor based tags. However, prior to the availability of printed active electronic components, the cost of integrating discrete passive components has prevented nonlinear tags from being used as low cost, disposable electronic article surveillance tags.
For a printed RF tag, the provision of a suitable substrate and/or an effective inductor coil can be a dominant factor in determining the cost of the tag. At 13.56 MHz and below, high Q inductors of a size <10 cm in lateral dimension (typically 50–100 μm thickness) require tens of microns of metal. High Q is generally required to get (1) good coupling between the reader field and the transponder tag and (2) high read range. Directly printing the nonlinear element on to a sheet or foil of metal can provide a cost effective way to provide an inductor, a relatively temperature resistant substrate, one electrode of the nonlinear device, and/or a source for the growth of the dielectric oxide.
As is known in the art, one can grow a dielectric film on a sheet of aluminum using high throughput, low cost per unit area processes (see e.g., U.S. Pat. Appl. Publication No. 2002/0163434, the relevant portions of which are incorporated herein by reference). However, a need still exists for low-cost or cost-effective integration of non-linear devices onto EAS RF tags. The present invention concerns a structure and process for an RF resonant and harmonic, subharmonic, signal mixing or sideband generating tag, utilizing printing technology.